1. Field of Invention
The present invention relates to a process for extractive distillation, and more particularly to a process for extractive distillation which reduces a solvent ratio by optimizing the temperature of solvent and the way of feeding in an existing extractive distillation column.
2. Description of Related Arts
Extractive distillation is utilized widely to separate an azeotrope or a liquid mixture consisting of several individual components having close boiling points in the chemical industries. The extractive distillation ordinarily feeds one or two solvents, which are soluble in the azeotrope or the liquid mixture, into the extractive distillation column to change or increase the relative volatility between the components to be separated, so as to separate components having close boiling points. As for the azeotrope, the solvent destroys the azeotrope system between the components, and makes the azeotropic point disappear, so as to separate the components. According to the way of feeding, the extractive distillation can be classed into continuous extractive distillation and batch extractive distillation. The continuous extractive distillation is more suitable to deal with a large amount of the azeotrope or the liquid mixture.
The solvents which have high boiling points and low volatility, do not form an azeotrope with the liquid mixture, and are hard to vaporize during distillation. The solvents are often fed from an upper portion of an extractive distillation column, while the mixture is fed from a lower portion thereof. In traditional extractive distillation, the mixture is saturated vapor when feeding, so as to make sure that the solvent concentration of the extractive distillation segment and the extractive stripping segment in the column is close, which helps separation of the components. However, the gas-phase feeding may cause serious local backmixing to the composition of components on the trays next to the feeding tray, and reduce separating effect of the extractive distillation column.
In the extractive distillation, the concentration of the solvent in the column mainly affects the relative volatility of the components and the separating efficiency. According to the relative art, the concentration is ordinarily 65%˜70%. The concentration of the solvent in the column is mainly determined by the solvent ratio and the reflux ratio (including the external reflux ratio and the internal reflux ratio). According to the traditional theory of extractive distillation, the solvent flowing downwardly contacts and absorbs the heavy components in the rising vapor feed, and leaves the light components rising up to be separated in the upper portion of the column. To enhance the contacting and absorbing effect, the column should provide a great deal of rising vapor feed, so a high column temperature or plenty of recycled feed is needed to maintain the concentration of solvent.
There was feeding the mixture as liquid. Parameters and structure of the extractive distillation column are adjusted and changed to cooperate with the way of feeding mixture. However, the adjusting and changing is according to the traditional theory, and still bases on providing a great deal of rising vapor, therefore can not reach an optimized condition and reduce the solvent ratio efficiently. Particularly, there is no previous practice that feeding mixture as saturated liquid in extractive distillation.
The temperature of the extractive distillation column is gradually increasing from top to bottom, and due to the high solvent ratio, the sensible heat of the solvent can not be ignored in the extractive distillation column. Ordinarily, the traditional theory requires that the feeding solvent has the same temperature as the liquid on the solvent feeding tray. Because if the temperature of the solvent is too high when feeding into the column, the rising vapor will be too much and break the liquid-gas balance in the column, and the separating effect will be affected. As a result, the external reflux ratio should be increased to maintain the liquid-gas balance, and a large solvent ratio is required to maintain the concentration of the solvent. For example, when the feeding temperature of the solvent is up to 64° C. in the first extractive distillation column of a C5 separation device in China, the solvent ratio is 8.6 wt/wt. It is obvious that merely increasing the feeding temperature of the solvent can not reduce the solvent ratio.
Accordingly, because the feeding solvent has the same temperature as the liquid on the solvent feeding tray, the temperature of the solvent increases as flowing down, and the solvent absorbs heat from the rising vapor feed, which causes that more vapor feed condenses down to increase the internal reflux ratio. In the circumstances, the solvent is diluted and the concentration thereof is reduced to affect the separating effect. To solve the problem, the prior art increases the solvent ratio to maintain the concentration of the solvent. For example, the solvent ratio of the first extractive distillation column of a butadiene extraction device is about 8 wt/wt, and the designed solvent ratios of the first and the second extractive distillation column of a C5 separation device in China are up to 8.8 wt/wt and 9.9 wt/wt respectively. The high solvent ratio reduces the efficiency of the trays and the column, and counteracts the effect of adding solvent which raises the relative volatility of the components and decreases the trays needed. As a result, the extractive distillation column costs more money and energy to build and operate.